well only took me six months to get around to taking pix. I forgot all about it. heh.
The seabag will work, but it's a little narrow. Straps sized / spaced for human shoulders vs sprawled across a spare tire. But if you are afield, rolling the mouth down a bit will hold it open an dstill leave enough material above the strap attachment points that the mouth folds closed. Instead of having to fiddle with latching the seabag closed. Works either way.
The shoulder straps have plenty to spare on the Sub's hatch, so they'll work on just about anything except maybe a pop-out rear window on a full-size van door. That might be too tall, but easy to sew some strap extensions on.
I would suggest that if someone has neither that they get the Large size ALICE pack. Roomier, more adjustability, bigger mouth, and three large pockets that could hold first aid, extra bags, field toilet kit, and the ALICE pack has all sorts of attachment points for things like an E-tool pouch. Both the seabag and the ALICE can be had for $20-30.
Since I still have a couple seabags from my service years, that's what I'm going with. 'Free' is good. And the only ALICE I still have around is a Medium and those don't have the volume of a seabag. And it's set up for other things, anyway.
The Seabagaroo: